Merge pull request #944 from showdownjs/fix/link-to-quickstart

fix: resolve broken link to installation guide

Author: bandantonio

docs/index.md

@@ -31,7 +31,7 @@ Showdown can be used on the client-side (in the browser) or server-side (with No
 
 ## Installation
 
-To install Showdown, follow the instructions from the [Installation guide](installation.md).
+To install Showdown, follow the instructions from the [Quickstart guide](quickstart.md).
 
 
 ## License